# Reciprocity

Reciprocity may refer to:

## Law and trade

* Reciprocity (Canadian politics), free trade with the United States of America
** Reciprocal trade agreement, entered into in order to reduce (or eliminate) tariffs, quotas and other trade restrictions on items traded between the signatories
* Interstate reciprocity, recognition of sibling federated states' laws:
** In the United States specifically:
*** Full Faith and Credit Clause, which provides for it
*** Concealed carry reciprocity
** Occupational licensing, which in some jurisdictions provides for it
* Traffic violations reciprocity where non-resident drivers are treated like residents
* Quid pro quo, a legal concept of the exchange of good or services, each having value

## Social sciences and humanities

* Norm of reciprocity, social norm of in-kind responses to the behavior of others
* Reciprocity (cultural anthropology), way of defining people's informal exchange of goods and labour
* Reciprocity (evolution), mechanisms for the evolution of cooperation
* Reciprocity (international relations), principle that favours, benefits, or penalties that are granted by one state to the citizens or legal entities of another, should be returned in kind
* Reciprocity (social and political philosophy), concept of reciprocity as in-kind positive or negative responses for the actions of others
* Reciprocity (social psychology), in-kind positively or negatively connoted responses of individuals towards the actions of others
* Ethic of reciprocity (the Golden Rule), that one should treat others as one would like others to treat oneself
* Serial reciprocity, in which the benefactor of a gift or service will in turn provide benefits to a third party

## Physical sciences and engineering

* Reciprocity (engineering), used in the analysis of structures and to resolve complex load conditions
* Reciprocity (electromagnetism), theorems relating sources and the resulting fields in classical electromagnetism
* Reciprocity (electrical networks), reciprocity theorem as it relates to current and voltage in electrical networks
* Reciprocity (network science), measures the tendency of vertex pairs to form mutual connections between each other
* Reciprocity (optoelectronic), a diode under illumination to the photon emission of the same diode under applied voltage
* Reciprocity (photography), the relationship between the intensity of the light and duration of the exposure that result in identical exposure
* Reciprocity of twist and wrench, in screw theory
* Reciprocity theorem (disambiguation), several unrelated results
* Reciprocity of antenna transmitting and receiving characteristics
* Helmholtz reciprocity, linear propagation.

## Mathematics

* Reciprocity law (law of reciprocity) in mathematics, including
** Quadratic reciprocity, a fundamental result in number theory
** Cubic reciprocity, theorems that state conditions under which the congruence x$^3$ ≡ p (mod q) is solvable
** Quartic reciprocity, a collection of theorems in elementary and algebraic number theory that state conditions under which the congruence x$^4$ ≡ p (mod q) is solvable
** Artin reciprocity law, a general theorem in number theory that provided a partial solution to Hilbert's ninth problem
** Reciprocity relation or exact differential, a mathematical differential of the form dQ, for some differentiable function Q
* Weil reciprocity law
* Reciprocal polynomials, the coefficients of the remainder polynomial are the bits of the CRC
* Reciprocal square root
* Reciprocity (projective geometry), a collineation from a projective space onto its dual space, taking points to hyperplanes (and vice versa) and preserving incidence
* Frobenius reciprocity, from group representation theory
* Reciprocity law for Dedekind sums
* Stanley's reciprocity theorem, states that a certain functional equation is satisfied by the generating function of any rational cone and the generating function of the cone's interior
* Hermite reciprocity for invariants of binary forms.

## Other

* Reciprocity (Fringe), a 2011 episode of the television series Fringe
* Rural Municipality of Reciprocity No. 32, Saskatchewan, Canada
* "Reciprocity", a song by Loudon Wainwright III from his 1976 album T Shirt